Titanium and titanium alloys are popular in the design of parts requiring a high strength-to-weight ratio and are particularly popular for parts to be employed in high temperature service, such as for jet engine parts. Titanium alloys for high temperature use require a fine grain size in order to enjoy improved mechanical properties over larger grained titanium alloys and in order to be inspected more efficiently. For example, when detecting internal defects by ultrasonic, non-destructive methods, the presence of large grains creates "background noise" or interference which generally requires rejection of the part. The presence of small grains, however, produces sonically quiet workpieces, that is, workpieces with minimum interference to sonic testing.
In certain applications, such as selected aerospace applications, certain manufacturers' specifications dictate that the grain size must not exceed 0.5 mm. Such limitations are associated with parts which, for example, are placed in high temperature service. In attempting to achieve fine grain size in titanium forgings, several processes exist, but none are directed to an isothermal forging process wherein .alpha. and .alpha.-.beta. titanium alloy bodies, such as bodies of Ti-6242 or Ti-17, are finish forged from a billet placed in an isothermal press to produce a workpiece having a maximum grain size not exceeding 0.5 mm. A discussion of each of these existing processes follows.
In U.S. Pat. No. 3,313,138, Spring et al disclose a process for forging billets of .alpha.-.beta. titanium base alloys. Spring et al utilize a V-die, rather than a flat die, and conduct the forging operation at a temperature below the .beta.-transus temperature of the .alpha.-.beta. alloy being worked. Spring et al teach that it is essential that a certain amount of work be done on the workpiece in the V-die forging step and state that it is essential that such step reduce the cross-sectional area of the workpiece by at least 10% or more, up to 50%, but preferably about 30%. In addition, Spring et al teach that it is possible to conduct some, or even most of the V-die forging step at temperatures above the .beta.-transus, so long as such forging is followed by forging below the .beta.-transus temperature to the extent of at least 10% reduction in cross-sectional area as a final part of the V-die forging step.
In U.S. Pat. No. 3,470,034, Kastanek et al disclose a process for producing a fine grained titanium alloy macrostructure which process involves heating an ingot or billet to a temperature between 50.degree. and 250.degree. F. above the alloy's .beta.-transus and then hot working, for example, by forging, the heated alloy as its temperature decreases to within a range of 50.degree. to 300.degree. F. below the alloy's .beta.-transus. This process is repeated in a cyclical manner producing progressively a finer grain size until a fine-grained titanium alloy macrostructure is achieved throughout the workpiece. This fine-grained macrostructure allows the material of the workpiece to be ultrasonically tested to exacting standards. Kastanek et al disclose that such a fine-grained macrostructure is necessary in order to reduce "background noise" and to produce sonically quiet billets, that is, billets with minimum interference to sonic testing.
In U.S. Pat. No. 3,489,617, Wuerfel discloses a method for processing bodies of .alpha. and .alpha.-.beta. type titanium base alloys which process involves refining the .beta.-grain size of .alpha. and .alpha.-.beta. type titanium base alloys and more particularly, involves refining the .beta.-grain size of such alloys during processing of ingots to billets for forging stock. Wuerfel's method consists of working a workpiece of the alloy from an initial temperature above the .beta.-transus to impart strain energy to the metal and recrystallizing the .beta.-grains. The recrystallization may be effected either simultaneously with working or by a separate anneal at a temperature at least as high as the initial working temperature. Specifically, Wuerfel teaches that his method must utilize an initial working temperature above the .beta.-transus of the alloy being processed and preferably will be between about 100.degree. and about 500.degree. F. above the .beta.-transus of the alloy. Wuerfel points out that at temperatures on the higher side of the range, dynamic recrystallization will occur simultaneously with working and will, therefore, take place throughout a large part of the working cycle, while at temperatures on the lower side of the range, an anneal at a temperature at or above the initial working temperature is required to effect recrystallization. Such an anneal generally will be between 2100.degree. F. and about 2400.degree. F., but must be at least as high as the initial working temperature. In Wuerfel's method, the anneal time is critical since it must be of sufficient duration to bring the metal body into the .beta. field throughout its extent. Wuerfel teaches that the anneal time will vary, for example, between about one hour and about four hours with the higher temperatures (e.g., those approaching 2400.degree. F., e.g., 2300.degree. F.) being employed with shorter time periods (e.g., those approaching one hour), and with the lower temperatures (e.g., those approaching 2100.degree. F.) being employed with longer time periods (those approaching four hours). Finally, Wuerfel teaches a single step process in which recrystallization is combined with working; the working must be initiated at temperatures substantially above the .beta.-transus of the alloy and that about 2200.degree. F. is the minimum for both the .alpha. and the .alpha.-.beta. type alloys with the preferred temperature range being from about 2200.degree. F. to about 2400.degree. F.
In U.S. Pat. No. 3,686,041, Lee discloses a process for producing ultra fine-grained titanium alloy microstructures which process involves heating the titanium alloy body to a temperature below the alloy's .beta.-transus temperature, but above its martensitic transformation temperature, hot working the heated alloy body as its temperature decreases, quenching, and repeating the cycle at least once. Lee does not teach, however, the heating of the titanium alloy above the .beta.-transus temperature.
In U.S. Pat. No. 3,635,068, Watmough et al disclose a method for bulk plastic deformation of titanium and titanium alloys utilizing elevated deforming temperatures in dies that are heated to or close to the workpiece temperature. The method taught by Watmough et al involves isothermal forming of the workpiece by heating the workpiece to a temperature above 1400.degree. F. and heating the dies to the same or a slightly lower temperature. The workpiece is preheated; the dies are heated by conventional heating methods, preferably by means external to the dies such as induction heating coils. Watmough et al disclose that the desirability of forming above or below the .beta.-transus depends upon the desired properties for the specific application of the alloy employed, and note that an important aspect of the process is control of the die speed during pressing.